noun.
Fixed and strong desire; esp. natural desire; a craving; an eager appetite.
noun.
Specifically: An instinctive inclination or propensity in animals to perform certain actions, as in the young to suck, in aquatic fowls to enter into water and to swim; the tendency of an organized body to seek what satisfies the wants of its organism.
noun.
Natural tendency; affinity; attraction; -- used of inanimate objects.